This invention relates to a method of operating a liquid crystal display cell filled with a smectic liquid. The method can be used to form a projection system in which a focused laser beam can be used to write bright lines on a dark background.
If a cell is constructed to confine a thin layer of liquid crystal exhibiting a smectic phase, and the interior surfaces of the cell are treated to promote homeotropic alignment of the liquid crystal molecules adjacent those surfaces, then the visual clarity of such a layer in the isotropic liquid phase is preserved as it is slowly cooled to transform it into the smectic phase. This clarity is preserved in the smectic phase because the slow rate of cooling enables the liquid crystal molecules to assume homeotropic alignment. If however the cell is cooled very rapidly the random orientation of the molecules in the nematic phase becomes transformed into a disordered orientation state in the smectic phase, with the result that the layer scatters light in the visible region of the spectrum.
The present invention utilizes these effects. It uses an intense flash of electromagnetic radiation to produce rapid heating followed by rapid cooling. This promotes the scattering state. In a projection system with a small numerical aperture such scattering regions are used to provide a dark field. Substantially white lines can be written on to this field by tracking a focused laser beam across the surface of the liquid crystal layer at an appropriate rate to supply sufficient energy to the regions successively illuminated by the focused beam to unlock the frozen-in scattering state and allow them to cool slowly enough to produce homeotropic alignment.